Run flat tires are designed to enable, even when the tires are punctured and air therein is lost, a running for a distance of about 100 km. Thus, by the use of the run flat tires, it is possible to avoid a situation where the vehicle is stopped at a dangerous place such as a road having a heavy traffic to change a punctured tire or to fix the puncture, so that the user is allowed to drive the vehicle to the nearest service station to perform maintenance on the vehicle safely. Furthermore, the run flat tires also eliminate the need to carry a spare tire on the vehicle, thus providing advantages such as an increased trunk space and improved fuel consumption by the vehicle having a lighter weight.
On the other hand, in the case of a tire like the run flat tire by which the vehicle can run even when the air pressure is zero, a conventional method focusing attention on a change in the tire rotation radius (see Patent Literature 1 for example) has a difficulty in determining a decrease in the air pressure. Thus, when the vehicle continues to run at a high speed with a decreased air pressure, the tires may burst.
Thus, in order to detect an abnormal air pressure of a tire when the air pressure of the tire is completely zero, there has been suggested a method in which the variations in the accelerations of the respective tires are compared (see Patent Literature 2 for example).
In the method according to Patent Literature 2, the variations in the acceleration of the respective tires are calculated and are compared. When the comparison result shows that a tire has an acceleration variation larger than those of the other tires, it is determined that the tire has an abnormal air pressure.
In the case of the method according to Patent Literature 2 however, a disadvantage is caused as described below. Specifically, when a snow chain is attached to a tire among four tires having a normal pressure, the acceleration variation calculated based on the rotation speed of the tire fitted with the snow chain is different from the acceleration variations calculated based on the rotation speeds of the other tires. Thus, the tire fitted with the snow chain is wrongly judged to have an abnormal air pressure, despite the fact that it has a normal internal pressure.
To prevent this, the present inventor has suggested an apparatus and a method by which a tire having a completely-zero air pressure can be accurately detected without causing a false alarm (Japanese Patent Application No. 2008-306125 which will be hereinafter referred to as “the invention of the previous application”).
The invention of the previous application was made based on the knowledge that, when one tire has a completely-zero air pressure, there is a case where the tire has a rotation speed higher than the rotation speeds of the other tires and there is also a contrary case where the tire has a rotation speed lower than the rotation speeds of the other tires and the degree of such delay in rotation speed is more significant than that in the case of the tire fitted with a snow chain. The invention of the previous application includes: a rotation speed detection means for periodically detecting tire rotation speeds of the respective wheels of a vehicle; an acceleration calculation means for calculating accelerations of the respective tires based on rotation speed information obtained from the rotation speed detection means; an acceleration variation calculation means for calculating acceleration variations of the respective tires based on the accelerations of the respective tires calculated by the acceleration calculation means; a comparison means for comparing the acceleration variation of the respective tires calculated by the acceleration variation calculation means; a presumption means for presuming, when the result of the comparison by the comparison means shows that a tire has a larger acceleration variation than the acceleration variations of the other tires, that the tire has a decreased air pressure; an examination means for identifying, based on the tire rotation speed information of the respective wheels, a tire having a rotation speed that is higher than the rotation speeds of the other tires or that is lower than a predetermined criterion; and a determination means for determining that a tire has a decreased air pressure when the tire identified by the estimation means is also identified by the examination means.
In the detection apparatus according to the invention of the previous application, the comparison means is used to compare acceleration variations of the respective tires. When the result shows that a tire has an acceleration variation larger than acceleration variations of the other tires, based on tire rotation speed information of the respective wheels, there is further detected a tire that has a rotation speed higher than the rotation speeds of the other tires or lower than a predetermined criterion. When a tire identified through the comparison of the acceleration variation is identical with a tire identified through the comparison of the rotation speed, a pressure decrease of the tire is determined. This can consequently avoid the generation of a false alarm in the case where only acceleration variations are compared. As a result, a status where the air pressure of one tire is completely zero can be accurately determined.
When a tire having a decreased pressure is determined only based on the acceleration variation of the tire, a case where a tire is fitted with a snow chain for example as described above causes a wrong determination in which the tire fitted with the snow chain has an abnormal air pressure in spite of the fact that the tire has a normal internal pressure. However, the present inventor has found that, when the air pressure of one tire is completely zero, the tire has a rotation speed higher than those of the other tires or, on the contrary, the tire has a rotation speed lower than those of the other tires. The present inventor also has found that the degree of delay in rotation speed of the tire is More significant than in the case of the tire fitted with a snow chain. In view of this, when a tire identified based on the acceleration variations of the tires is identical with a tire identified based on the rotation speeds of the tires (Namely, a tire having a speed higher or remarkably slower than those of the other tires. The degree of “slowness” in speed of the tire is much larger than that of a tire fitted with a snow chain.), it is determined that the tire has a decreased air pressure. This can consequently prevent the apparatus from determining wrongly that a tire fitted with a snow chain is “decreased in the air pressure” when the tire has an increased acceleration variation.